Method and apparatus for picture transmission



oct. 13, 1942. R D, KELL 2,298,799

METHOD ANDVAPPARATUS FOR PICTURE TRANSMISSION Suvenor f d, GttorncgFifa. 1.

R. D. KELL METHODAND APPARATUS FORPICTURE TRANSMISSION Filed Aug. 31,1939 5 Sheets-Sheet 2 EtccEEt l Hlllu" mkum. GENS "Illu Oct. 13, 1942.R. D. KELL. 2,298,796

METHOD AND APPARATUS lFOR PICTURE TRANSMISSION Filed Aug. 3l, 1959 '5Sheets-Sheet 3 faz: 6L'

/f pag.5115' Enventor `Haya Kell a (Ittorneg vJl Oct. 13, 1.942.

R. DQ .Kl-:LL 2,298,796

METHOD AND APPARATUS FOR PICTURE TRANSMISSION Filed Aug. :51, 19:59

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Patented Get. 13, 1942 METHOD AD APPARATUS FOR PICTURE TRANSMISSION RayD. Kell, Haddonfield, N.J., assignor to Radio Corporation of America, acorporation of -Dela- Application August 31, 1939, Serial No. 292,925

(ci. 17a-7.2)

7 Claims.

My invention relates to the transmission of pictures and has for itsobject to provide an improved. method of and means for so transmittingpictures from motion picture film or the like that the picture may bereproduced at the receiver with the proper background or average overallillumination. According to one aspect of the invention, it provides asystemwherein the direct current component of the video signal may bereinserted at any point in the system in cases where this component hasbeen lost as a result of passing the signal through alternating current,According,

amplifiers, transformers, or the like. to another aspect of theinvention, it makes possible thetransmission of video signals includingthe direct current component through an alternating current amplifier orthe likewithout any change in the location of their alternating cur- Inapplying a preferred embodiment of my invention to a televisiontransmitter, there is employed a cathode ray picture transmitter tube ofthe type recently described by Iams and Rose. The main feature of thistube is that the electron beam velocity is so low at the point of impactat the mosaic .that the secondary emission ratio is less than unity. Ihave observed that when motion picture film istransmitted by projectingthe picture frames upon the cathode ray tube mosaic A during the returnlineperiods onlyand then vgsa'nning, the ,mosaic after the light hasbeen cut off; the'sign'a'l level representative of black in the picturecorresponds "exactly with the position of the A. C. axis of the signal.'Ihe reason-for this is that the signal produced during a return lineperiod (during which period a light image is projected upon the mosaic)is a result of electrons leaving the whole mosaic in proportion to thelight and shade of the picture, while that produced during the usefulscanning period is a result of the same number of electrons beingreplaced on the surface ofthe mosaic. Stated in another way, theintegrated signal amplitude with respect to time during the imageprojection period is exactly equal to the integrated signal amplitudewith respect to time during the useful signal period. e Y

Thus, it will be apparentvthat a signal of the above-described charactermay be transmitted through a system including alternating currentamplifiers without any shift in the position of the A. C. axis. In otherwords, the D. C. component of the signal would not be lost.

However, because of the large amplitude of the signal pulse producedduring the return line period, it is preferred to Aclip off the greaterpart ofit in an early amplier stage and to utilize the resulting signallater for reinserting the direct current component.

The invention will be better understood from the following descriptiontaken in connection with the accompanying drawings in which .Figure 1 isa diagram showing one embodiment ofthe invention as applied to atelevision transmitter,

Figure 2 is a circuit diagram the transmitter shown in' Figure 1,

Figures 3, 4, 5 and 6;' are curves which are referred to inexplaining'1V the invention,

Figure 7'is a diagramv showing the clipping action which occurs in thecircuit shown in Figure 2,

Figure 8 is a curve illustrating the character of the video signal whichis transmitted to the receiver, l

Figure 9 isa circuit diagram of a portion of a television transmitterillustrating another embodiment of my invention, and e Figures 10 and 11are. diagrams whichare referred to in explaining the operation of thecircuit of Figure 9.

Referring to Fig. 1, the cathode ray picture transmitting or pickup tubeis shown at I. This tube, which is ofthe type wherein the electronvcontrol electrode 3 and accelerating electrodes 4 and 6. Suitablebiasing and accelerating voltages are provided by the'batteries 'I and8.

By means of a focusing coil 9, theelectron beam leaving the electron gunis focused to a small spot on a mosaic screen II. The screen II is ofthe well-known type commonly employed in iconoscopes with the exceptionthat it is made translucent-I whereby the image of a frame of the motionpicture film I2 to be transmitted may be projected on the photoelectricelements of the screen from the back side by means of a suitable opticalsystem indicated at I3.

The electron beam is caused to scan the screen of a portion of vtheRadio provided for electrostatic shielding purposes. A

centrally apertured electron-collecting electrode I9 is provided towhich the electrons of the beam not reaching the mosaic screen II aredirected`v` and collected.

As explained in the above-mentioned Rose patent, the electrons of theelectron beam are directed upon the mosaic screen II with a very lowvelocity, that is, a velocity approaching zero velocity at the point ofimpact therewith. The reason for employing this low velocity is that nthe electrons of the beam should approach the point of impact on themosaic with -a velocity such that the ratio of secondary electrons toprimary electrons is less than unity.

In operation, elemental areas of the mosaic electrode acquireelectrostatic potentials proportional to the intensity of light incidentthereon. Particles of the mosaic which are more highly illuminatedacquire the most positive electrostatic charge with respect to theunilluminated particles. The positive charges representing anelectrostatic image of a picture to be transmitted are neutralized bythe scanning beam electrons. When the electrons of the scanning beam aredirected toward those particles of the target which are negative withrespect to the cathode, they cannot reach those particles because oftheir low velocity. These electrons, since they lare prevented fromimpinging on the target are returned to an electron collecting electrodeadiacent the electron gun. The scanning means which causes th'e electronbeam to scan the mosaic electrode or target are so chosen as to preventthose electrons of the beam which do not reach the target from returningto the electron gun but rather causes them to be redirected along pathsother than those `followed by those electrons in traversing theAdistance between 'the electron gun and the target.

A tube of the above type has a linear characteristic between light inputand signal output. It is so adjusted that when the mosaic screen II isdark all of the electrons of the electron beam are repelled and returnedto the collector electrode I9. Any light on the screen II causes some ofthe electrons of the beam to reach the screen for neutralizing thepositive charges which the light has caused the photoelectric capacityele- 55 ments to assume. Th'is neutralizing current is the picturecurrent. Preferably, it is taken off the back or signal plate of thescreen II and supplied to a picture amplifier 45 through an 6 outputresistor 30.

Referring now more specifically to the circuit associated with thepickup tube, there is provided a suitable synchronizing signal andblanking impulse generator indicated at 26. This generator preferablysupplies synchronizing and blanking signals similar to those describedin Bedford Britishl Patent 448,065. It may comprise rotatable discshaving apertures therein corresponding to the impulses desired asdescribed ln-the said British patent, or it may comprise vacuum tubeoscillators and shaping circuits as described in Smith Patent No'.2,132,655.

ing circuit 28 which forces the desired saw-tooth current through thedeecting coils I6.

Likewise, synchronizing impulses occurring at the horizontal deiiectingfrequency are supplied over a conductor 29 to' a, suitable horizontaldeflecting circuit 3I which impresses a saw-tooth deilecting voltageacross the delecting plates I4,

The basic method of transmitting the motion picture film is thatdescribed in my Patent No. 2,166,214, issued July 18, 1939. Preferably,the system employs the so-called 2-3 intermittent mechanism described inBedford Patent No. 2,082,093, which' makes possible the transmission ofstandard 24-frame-per-second iilm at a scanning rate of 60 frames persecond.

The illm projector and associated apparatus comprises a film gate 32through which the motion picture lrn i2 is drawn by means of anintermittent mechanism 33, preferably of the type described in BedfordPatent No. 2,082,093. This mechanism is driven by a synchronous motor 34operated from the 60-cycle power line. The motor also drives a shutterdisc 36 having an opening therein through which an image of a pictureframe may be projected upon the cathode ray tube mosaic i I during thereturn line period, In the specic example illustrated, the motor 36operates at 3600 R. P. M., and the shutter disc has only one shutteropening wh'ereby the shutter opens 60 times per second.

Obviously, the electron beam of the cathode t ray tube I must bedeflected in the proper time relation to the projector and shutteroperation.

This time relation is insured by the use of a control circuit 31 whichholds the impulse generator 26 in a fixed time relation to the 60-cyclepower supply. Such' a control circuitis described in Bedford Patent No.2,137,010.

Preferably, vertical and horizontal blanking impulses are supplied fromthe generator 26 through an amplifier and a coupling condenser to thecontrol electrode 3 ofthe tube I, these impulses having the properamplitude, polarity and timing to block the electron beam during 5 boththe vertical return line time and during the synchronizing or drivingimpulses occurring at 4 the vertical deiiecting frequency are suppliedover a conductor 21 to a suitable vertical deflecthorizontal return linetime.

The video signal appearing across th'e output resistor 30 is of thecharacter shown in Fig. 3 where the signal produced by the picture framebeing fiashed or projected upon the mosaic is indicated at while thepicture signal produced as the electron beam scans the mosaic isindicated at y. The horizontal blanking pulses cause th'e output signalto go to black periodically as indicated at z. It may be noted that, inorder to simplify the drawing, ,only a small number of horizontal pulses2 have been indicated. The

area of the signal a: above the A. C. axis is always the same as that ofthe signal y below the axis.

0 This is because, in the cathode ray tube used in the electron beamduring the scanning period merely replaces the electrons that werereleased from the mosaic when the picture was projected upon it.

Thus, if the scene becomes darker, the amplitude of both signal portions:c and y decreases. The result is that the A. C. axis is always at thelevel representing black in the picture.

This is further illustrated in Figs. 4, 5 and 6. If the mosaic screen isdark, there is no signal, this condition being illustrated in Fig. 4. Ifthe screen is uniformly illuminated to an intensity representing gray,the signal is as representedin Fig, 5. If the illumination is increasedto the maximum illumination representing white, the

this system,

signal is as shown in Fig. 6. It will be seen that the A. C, axis hasnot shifted, but has remained at th'e level representing black in thepicture.

From the foregoing, it will be evident that the cathode ray tube outputmay be transmitted to the cathode ray receiver tube through A. C.amplifiers without any loss of the direct current component ofthepicture. This procedure, however, has several disadvantages, onebeing that the amplitude of the signal produced by the light flash onthe mosaic is larger (about ten times that ofthe picture signal, forexample) whereby amplifiers are quickly overloaded, and another beingthat there is no way of transmitting vertical synchronizing signals ofthe desired characteristics.

Accordingly, it is preferred to clip olf either all or a substantialportion of the impulse portion a: of the video signal at a suitablepoint either in the preamplifier 45 or in the video amplifier 50. -Forexample, as illustrated in Fig. 2, an early stage 31 of the videoamplifier may be given a sufiicient negative bias to clip off the signalportions :c at the black level. This clipping action is shown in Fig. 7where the characteristic of the tube is shown at 39. The video signalhaving the character illustrated in Fig. 3 4

is applied to the clipping tube with the pulses :c in the negativedirection.l The pulses in the resulting signal appearing in the platecircuit of the clipping tube have a fixed amplitude at the black leveland a varying height with respect to the alternating current axis. Aswill be described later, it may be preferred to rst clip o serted at anydesired point orpoints in the system. Preferably, direct currenttransmission is employed for more efficient operation in which case thedirect current is reinserted at or close to the modulation stage of thetransmitter. In this case, as well as where A. C. transmission isemployed, the D. C. component is'reinserted in the receiver at thecathode:,r ay tube to provide automatic background eontrol, sincethevideo amplifiers in the receiver are usually of the A. C. type forbest operation.-

Merely by way of example, there isv shown in Fig. 2 a D. C. reinsertingstage for D. C. transmission. This particular stage comprisesl anamplier tube 4| to which the video signals are fed through a suitable.umber of amplifiers which have been omitted from the drawings asindicated by the. broken line and through a grid condenser 42, thesignals being appliedlto the grid of the tube 4I with the synchronizingpulses of positive polarity. The grid leak resistor 43 has. such resistance with respect to the capacity of the grid condenser 42 that, dueto the synchronizing impulses driving the grid positive periodically,there is produced a grid leak biasing voltage that varies in' accordancewith the height of the synchrof nizing pulses as measured from theV A.-C. axis.4

-. At the receiver, the D. C. may bereinserted after having been removedby the receiver video Aamplifier or amplifiers by means of the same l0`type of D. C. reinserting stage as that shown in Fig. 2 or by anyone ofseveral other reinserting circuits such as the circuits shown in HolmesPatent 2,251,677, issued Aug. 5, 1941, led February 28, 1933, entitledTelevision systems, and assigned to the Radio Corporation of America.

This Holmespatent claims broadly the feature of reinserting the D. C.component in accordance with the variation in height of thesynchronizing impulses.

Referring more specifically to that portion of the circuit whichcombines the picture signals with the blanking and synchronizingimpulses, it will be seen by reference to Fig. l that with a switch 56closed the blanking impulses are applied to a stage of the videoamplifier 50. Also, the synchronizing pulses are applied through anamplifier 51 to some stage in the video amplifier 50.- For examplereferring to Fig. 2, the picture synchronizing and blanking signals maybe added at the same point in the circuit by applying them to theamplifier tubes 58, 59 and 6I, respectively, which have a common plateresistor 62.

The above-described apparatus may be 4operated with the switch 56(Fig. 1) open whereby the synchronizing pulses are set directly on thepicture signal at the black level. It is preferred, however, to have thepedestal or blanking portion of the final signal extend slightly beyondblack as shown in Fig. 8 in order that the adjustment at thecathode raytube of the receiver will not be critical for obtaining good blanking,it being understood that at the receiver tube the blanking portion ofthe signal drives the tube to beam cut-off.

Instead of clipping the signal pulses :c at the black level as describedabove, it is generally preferred to clip them somewhat above the 'blacklevel as shown in Fig. 10. The reason this is preferred is that, unlessspecial tubes are employed, the clipping should be done in an -earlyamplifier stage where the signal voltage is low, in order to avoidsaturating ofi` any of an impulse as. On the other hand, vacuum trbechar-` acteristics are such that good clean clipping is difficult toobtain at low signal voltages.

It will be apparent that, when clipping is cone above the black level,there must be a second clipping step in order to bring the verticalpedestais to the same level (the black level) as that of the horizontalpedestals z. Since this second clipping step is necessary, it isimmaterial that the first clipping of the signal is not as complete as'required in the final signal.

In Fig. 9 there is shown a video amplifier which Vmay be substituted forthe .ideo amplifier 50 of Fig. 1 and Fig. 2. Like parts. in thesefigures are indicated by the same reference numerals. The first'clipping of the signal may be done by the .tube 31 with less lbias onthe control grid than that indicated in Fig. 7. As a result, the

pulses :c are clipped beyond the black level in the` black direction, asindicated in Fig. 10. `It may be preferred in some cases to perform thisfirst clipping step in one stage of'the preamplifier 45,

since it should be done where the large pulses a: do not saturate theampliiier. The important point is that at the rst clipping stage the A.C. axis of the signal should be located at adeilnite level such as theblack level, as distinguished from some indefinite location which wouldresult if portions of the pulses :v were saturated off before the iirstclipping.

After being amplified in one or more stages. the clipped signal isclipped the second time, as described above, by the tube 58 which has avariable bias applied thereto by means of a diode 66 and its associatedcircuit elements. The cathode of the diode 66 is connected to thecontrol grid terminal of the tube 58, while the diode plate is connectedto ground, preferably through a biasing battery 61. With thisconnection, negative pulses will cause a flow of diode current whichcharges the grid condenser 68. This charge leaks oir through a grid leakresistor 69. The relative values of elements 68 and 69 are such that thedischarging circuit of condenser 68 has a long time constant as comparedwith the frequency of occurence of the pulses :c whereby the dischargeof condenser 68 applies a bias to tube 58 which follows variations inthe height of the clipped pulses :c as measured from the A. C. axis ofthe signal. 1l. It will be seen that the diode introduces a bias voltagewhich holds the peaks of the pulses :z: at a fixed level such that theyare clipped oli at the desired black level. i

The output of the tube 58 as shown in Fig l1 is of the same character asthat shown in Fig. '7, where the first and only clipping operation wasat the black level. Therefore, it may have the blanking andsynchronizing pulses added theret in the manner previously described toproduce a nal signal like-that shown in Fig. 8.

While no D. C. reinserting stage has been shown in Fig. 9, it will beunderstood that such a stage may be employed as previously described.

From the foregoing, it will be apparent that various modifications maybe made in my invention without departing from the spirit and scopethereof, and I desire, therefore, that only such limitations be imposedthereon as are necessitated by the prior art and are set forth in theappended claims.

I claim as my invention:

1. A picture-transmitting system comprising in combination a cathode raytransmitter tube of the type in which there is a mosaic screen hav- Aingelemental capacity elements which are electron-emissive and in which anelectron beam is directed toward said screen, the electrons at the pointof impact on said screen having a velocity such that the ratio ofsecondary electrons emitted from said screen to primary or beamelectrons is less than unity, means for projecting images of a pictureor scene upon said screen intermittently at a certain rate wherebyperiodically recurring signal pulses of a certain polarity appear in theoutput circuit of said tube in synchronism with said projection, meansfor scaning said screen with the electron beam at the end of eachprojection whereby lpicture signals of opposite polarity to said certainpolarity appear in said output circuit, the resulting video signal insaid output circuit having the characterstic that the alternatingcurrent axis is always at the level representing black in the picture,clipping means having a fixed bias for clipping oi said signal pulses atsaid black level whereby pedestals are produced, means for producingperiodically recur- This action is shown in Fig.

Iing said synchronizing pulses to said clipped video signal that theyextend beyond said pedestals in the direction of black.

2. A picture-transmitting system comprising in combination a cathode raytransmitter tube of the type in which there is a mosaicscreen havingelemental capacity elements which are electron-emissive and in which anelectron beam is directed toward said screen, the electrons at the pointof impact on said screen having a velocity such that the ratio ofsecondary electrons to primary electrons is less than unity, means forprojecting images of a picture or scene upon said screen intermittentlyat a certain rate ,whereby periodically recurring signal pulses of acertain polarity appear in the output circuit of said tube insynchronism with said projection, means for scanning said screen withthe electron beam at the end of each projection whereby picture signalsof opposite polarity to said certain polarity appear in said outputcircuit, the resulting video signal in said output circuit having thecharacteristic that the alternating current axis is always at the levelrepresenting black in the picture, means for clipping oi said signalpulses at a fixed level beyond black, means for next clipping off saidsignal pulses at said black level whereby pedestals are produced, meansfor producing periodically recurring synchronizing pulses that occursimultaneously with said pedestals, and means for so adding saidsynchronizing pulses to said clipped video signal that they extendbeyond said pedestals in the direction of black.

3. A picture-transmitting system comprising, in combination, a cathoderay transmitter tube of the type in which there is a mosaic screenhaving elemental capacity elements which are electron-emissive andinwhch an electron beam is directed toward said screen, the electrons atthe point of impact on said screen approaching zero velocity, meanscomprising an intermittent mechanism and shutter for projecting imagesof a picture or scene upon said screen intermittently at a certain ratewhereby signal pulses of a certain polarity appear in the output circuitof said tube in synchronism with said projection, means for scanningsaid screen with the electron beam at the end of each projection wherebypicture signals of opposite polarity to said certain polarity appear insaid output circuit, the resulting video signal in said output circuithaving the characteristic that the alternating current axis is always atthe level representing black in the picture, means for clipping off saidsignal pulses at said black level whereby pedestals are pro-y duced,means for producing periodically recurring synchronizing pulses thatoccur simultaneously with said pedestals, and means for so adding saidsynchronizing pulses to said clipped video signal that they extendbeyond said pedestals in the direction of black.

4. A picture-transmitting system comprising in combination a cathode raytransmitter tube of the type in which there is a mosaic screen havingelemental capacity elements which are electron-emissive and in which anelectron beam is directed toward said screen, the electrons at the pointof impact on said screen approaching zero velocity, means comprising anintermittent mechanism and shutter for projecting images of a picture orscene upon said screen intermittently at a certain rate whereby signalpulses of a certain polarity appear in the output circuit of said y tubein synchronism with said projection, means for 'scanning said screenwith the electron beam signals of opposite polarity to said certainpolarity appear in said output circuit, the resulting video signal insaid output circuit having the characteristic .that the alternatingcurrent axis is-always at the level representing' black in the picture,means for clipping off said signal pulses ata iixed level beyond blackbefore. any substantial' distortion of the video signal has beenproduced, means' for lnext clipping off said signal pulses. yat saidblack level whereby pedestals extending to said black level areproduced,means for producing periodically recurring synchronizing pulsesthat occurl simultaneously with said pedestals, and

` means for so adding said synchronizing pulses to the video signalafter said second clipping that' theyextend beyond -said pedestals'inthe direction of black.

` 5. ,A picture-transmitting Asystem comprising,

l' in combination, a cathode raytransmitter tube of the type in whichthere is a mosaic screen hav- `ing elementalcapacity elements-which areelec` tron-emissive andin which an electron beam is i directedtowardsaidscreen, the electrons at the point of ,impact onsaid.screeniapproaching zero velocity, means comprising an intermittentmechanisnil and shutter for projectingimagesofa picthe point'of impacton said screen approaching zero velocity, means comprising anintermittent mechanism and shutter for projecting images oi a picture orscene upon said screen intermittently at a certain rate whereby signalpulses of a cerltain polarity appear in the output circuit of said tubein synchronism with said projection, means for scanning said screen withthe electron beam at 'the end of each projection whereby picture signalsof opposite polarity to'said certain polarity appear in said 4outputcircuit, the resulting video signal in said output circuit having thecharacteristic that the alternating current axis is al- Ways at thelevel representing black inthe picture, means for clippingl oir saidsignal pulses at a iixed level beyond black before any substantialdistortion of the video signal has been produced by ampliner saturationor the like, means for amplifying said clipped signal, means for nextclipping oir said signal pulses at said black llevel whereby `pedestalsextending to said black level are produced, meansfor producingperiodically recurring blanking pulses that occur simultaneously `withsaid pedestals, means for also producing periodically recurringsynchronizing pulses that fccur simultaneously with said pedestals, andmeans fcrso-adding said blanking and syn- 'chronizing pulses tothevideosignal after said second clipping that said blanking and synchronizinglpulses extend beyond said black level in the directionof black with`the synchronizing pulses set on top of. said blanking pulses.

polarity appear in the output circuit of said tubey in synchronism withsaid projection,means for scanning said screen with the electron beamat' the end of each projection whereby picture signais of oppositepolarity to said certain polarity appear in said foutputcircuit, theresulting video signal in said output circuit havingthe characteristicthat the alternating current axis is 'always at the level representingblack in the picture, means for clipping oil said signal pulses at'v axed level beyond black before any substantial distortion of the videosignallias been produced by amplifier saturation or the like, means fornext clipping off said signal pulses at said black level wherebypedestals ,extending to said black level are produced, means for alsoproducing periodically recurring blanking and synchronizing pulses thatoccur simultaneously with said pedestals, and means for so adding saidblanking and synchronizing pulses to the, video signal after. saidsecond r clipping that said blankingand synchronizing pulses extendbeyond said' black level in the direction of black with thesynchronizing pulses set.

on top of said blanking pulses.

6. A picture-transmitting system comprising, in combination, a cathoderay transmitter tube of the type in whichhthereis a mosaic screen havingelemental capacity elements which are n electron-emiss'ive and in whichan electron'beam is directed toward said screen, the electronsat 7. Themethodv of transmitting pictures in a picture-transmitting `system ofthe type including a cathode ray transmitter tube having a mosaic screencomprising elemental capacity elements which are electron-emissive andhaving an electron beam directed toward said screen, the electrons ofsaid beam at the point of impact on said screen havinga .velocity suchthat the ratio of secondary electrons to primary electrons is less thanunity, said method comprising projecting images of a picture or sceneupon said screen intermittently at a certain 'rate whereby periodicallyrecurring signal pulses ofa certain polarity appear in the outputcircuit ofsaid tube in synchronism with said projection,4 scanning saidscreen with said electron beam 'at the end of each projection wherebypicture signals of oppositev polarity to said certain polarity appear insaid4 output circuit, the .resulting video signal in said output circuithaving the characteristic that the alternating current axis `is alwaysat the level representing black in the picture, clipping off said Asignal pulses at said black level whereby pedestais areproduced,producing periodically recurring synchronizing' impulses thatoccur simultaneously with said pedestals, and so adding saidsynchronizing impulses to said clippedvideo signal that they extendbeyond said pedestals in the direction of black.v j I RAY. D. KELL. i

